An image forming apparatus employing an electrophotographic process such as a copying machine, a printer, or a multi-function peripheral (MFP) has an optical scanning apparatus for scanning a light beam modulated according to a level of image data in a main scanning direction of a photoconductive drum.
In the optical scanning apparatus, various cylindrical lenses are used in order to form a beam from light output from a light source. The cylindrical lenses include a cylindrical lens of a plano-convex type, one surface of which is a plane and the other surface of which is formed in a convex shape, a cylindrical lens of a plano-concave type, one surface of which is a plane and the other surface of which is formed in a concave shape, a cylindrical lens of a bi-convex type, both surfaces of which are formed in a convex shape, and a cylindrical lens of a bi-concave type, both surfaces of which are formed in a concave shape. The light passes through the surfaces. These cylindrical lenses may be independently used or some of the cylindrical lenses may be used in combination.
When the cylindrical lenses are arranged on an optical path of the scanning optical system, it is necessary to accurately match a direction of the optical path and a direction of an optical axis of the cylindrical lenses. Positioning of the cylindrical lenses (in the following explanation, positioning includes angle adjustment for the cylindrical lenses) is extremely important.
In the cylindrical lenses of the plano-convex type and the plano-concave type, each one of the surfaces is a plane. Therefore, these cylindrical lenses can be relatively easily positioned by providing reference surfaces in bases on which the cylindrical lenses are mounted and matching the planes of the lenses and the reference surfaces of the bases.
In the case of the cylindrical lens of the bi-convex type, if accuracy of a bottom surface thereof is high, it is possible to position the cylindrical lens by pressing a convex surface on one side against a reference plane provided in a base.
In contrast, in the cylindrical lens of the bi-concave type, although a plane serving as a reference can be provided on a bottom surface, a plane serving as a reference is not present in an optical axis direction of the lens. Therefore, conventionally, when the cylindrical lens of the bi-concave type is mounted on a base, a method of performing optical axis alignment by pressing both edges of a concave surface against a reference surface provided perpendicularly to the base is often adopted.
However, in particular, in a cylindrical lens of the bi-concave type formed of a glass material, since a crack and a chip tend to occur at edges of a concave surface, the cylindrical lens cannot be held with sufficiently strong force. When a crack or a chip occurs at the edges, the cylindrical lens may not be able to be positioned at a right angle.
In order to prevent a crack and a chip of the edges, a method of chamfering the edges is often adopted. However, in a chamfering method in the past, even after chamfering is performed, an edge portion projecting in an optical axis direction of the cylindrical lens remains. Therefore, highly accurate positioning cannot be expected.